1. Field of Invention
The present invention relates to a photometric apparatus for use in auto-focusing cameras which employs a charge accumulation type photoelectric transfer means which obtains a photometric value using the photoelectric transfer outputs and the charge accumulation times of normally used charge accumulation type photoelectric transfer elements. More particularly, the present invention pertains to a photometric apparatus which is capable of obtaining a highly accurate photometric value in a short period of time under a flickering light source, such as a fluorescent light.
2. Related Background Art
In conventional autofocusing cameras, it is impossible to obtain a highly accurate photometric value regarding the brightness of an object from the output of a focal point detection device structured by normally employed charge accumulation type photoelectric transfer elements under a flickering light source, such as a fluorescent light, because the photometric value obtained from the charge accumulated at the crest or trough of a flicker varies from the average brightness of the object.
Japanese Patent Application Laid-open Nos. 62-19824 and 62-259022 disclose a structure which is capable of eliminating influence of variations in the photometric value in a plurality of charge accumulation operations by using the average value of the charge accumulation time determined by a real-time monitor (hereinafter, this method is referred to as a hard auto gain control, and the charge accumulation time determined by this method is referred to as a hard auto gain control time) to obtain an accurate photometric value of the average brightness of an object. Here, the charge accumulation operation indicates series of operations including the charge accumulation, charge transfer, A/D conversion, and arithmetic operations.
In the focal point detection device structured by the charge accumulation type photoelectric transfer elements, a method of determining the charge accumulation time from the output obtained in the previous charge accumulation operation (hereinafter, this method is referred to as a soft auto gain control, and the charge accumulation time determined by this method is referred to as a soft auto gain control time) is also known.
In the apparatus driven by the soft auto gain control method, the charge accumulation output can be set to an optimum level independent of distribution of the brightness of the object.
However, in the apparatus driven by the soft auto gain control method, the charge accumulation output may be saturated due to the influence of the flicker of the light source. That is, if the previous charge accumulation operation is conducted at the trough of a flicker, i.e., if the previous charge accumulation operation is conducted in a dark state, the charge accumulation time for a subsequent charge accumulation operation (which means the present charge accumulation operation) may be set to a time longer than an actual time. If the present charge accumulation operation is conducted at a portion which is not the crest of the flicker, unlike the previous charge accumulation operation, the charge may be accumulated too much, saturating part (a bright portion) or the entirety of the charge accumulation output.
Saturation of the charge accumulation output means that there is no data on the brightness of the saturated portion. It is therefore undesirable to obtain a photometric value using such data.
In the above-described structure driven by the hard auto gain control time, the monitor element for controlling the charge accumulation time of the focal point detection device on a real time basis (normally a silicon photo diode) generally has a far smaller area than the light-receiving area of the pixel element groups of the apparatus which employs the monitor element. Therefore, the use of only the hard auto gain control time determined by the output of such a monitor element as the data on the brightness of the object deteriorates the accuracy of obtained photometric value.
Regardless of the type of the auto gain control, a charge accumulation operation requires the A/D conversion time which lasts 20 milliseconds to 50 milliseconds and the focal point detection time, in addition to the charge accumulation time and the charge transfer time. Therefore, quick photography of the camera is sacrificed by conducting the charge accumulation operation a plurality of times to enhance the photometric accuracy (four times in the case of Japanese Patent Application Laid-open No. 62-259022).